Sensory Systems

Sensory “receptors”

• In order to respond to your environment, or maintain homeostasis, you must first “sense” that stimulus– Sensation = awareness of a stimulus

• The brain becomes aware that there is light because photoreceptors send this signal to the brain

– Perception = conscious awareness of sensations• More “higher” level integration/association

involved in perception– Light is yellow, it’s in the shape of a “c”…it’s a banana

Sensation

Perception

Sensory “receptors”• You have sensory “receptors” for each form of

stimulus– Sensory receptors have some unique traits

• While most are neurons, some in fact are specialized cells that can communicate with neurons by releasing neurotransmitter

– Remember that every cell with a nucleus will have the same DNA…every cell can therefore produce ANY protein that you’d likely find in your body

– Many sensory cells can “adapt”• Gradually reduce their action potentials if continuously stimulated

(sharp pain – dull pain)• Gas station attendants who don’t smell the gas etc.

– Other sensory cells do not “adapt”• Will continue to send action potentials as long as the stimulus is

present– Bright light

Sensory “receptors”• “Adaptation”

– Sensory receptors that reduce their action potentials when constantly stimulated are said to be undergoing “receptor desensitization”

• Are desensitizing themselves to the stimulus• Can be through 2 distinct processes:

– Hyperpolarizing themselves…creating a more strong membrane potential (MORE positive outside, MORE negative inside)… “supercharging” the battery across the plasma membrane

– Reducing the number of receptors on their surface» Endocytose regions of neurolemma or plasma

membrane into the cell body…reduces their exposure to their external environment

Sensory “receptors”• Mechanoreceptors: fire action potentials in response to

distortion of the cell or the receptor on that cell

• Thermoreceptors: fire action potentials in response changes in temperature– You have more “cold” receptor cells than “hot” receptors cells in your

skin

• Photoreceptors: fire action potentials in response light or color – In your eye

• Chemoreceptors: fire action potentials in response to changes in their extracellular chemical environment– Osmo-receptors in your brain constantly check the osmolarity of blood

• Pain receptors: fire action potentials in response to damage to nearby tissue

Sensory “receptors”• Receptor cells can also be divided into:

– General senses: throughout the body• Often utilize single receptor cells, or clusters of

receptor cells

– Special senses: retained in skull (close to the brain)

• Utilize “specialized” sense organs – Taste, smell, sight, hearing & balance

Mechanoreceptors

MechanoreceptorsIn skeletal muscle, and within the tendons that attach muscle to bone, there are specialized mechanoreceptors. These “proprioceptors” allow the brain to monitor the result of the motor outputs.

The “proprioceptors” accomplish this through a combination of specially-shaped muscle cells, and the neurons that wrap themselves around these “spindle” cells.

Thermal receptors• Thermal sensation usually close to the

surface of the skin– Normally a free neuron (not wrapped in any

special tissue)– Classic example of “adaptive” receptors

• Hot shower gradually warms “down” because thermoreceptors are reducing their action potential rates

– Unless you run out of hot water

– Recall that you have separate cold & hot receptors

• Numerically, have more cold receptors than hot

Pain receptors• Pain receptors are often called “free nerve

endings”– Neurons that have no special “structural” sensor

interface…just a bundle of dendrites spread over a general area

– Pain receptors in the skin (integument) are usually close to the surface of the skin

• Often in the same location as thermoreceptors for cold & heat sensation

Pain receptors• Bear in mind that pain receptors are neurons

that can sense Substance P– The surrounding tissue will release Substance P

(substance “pain”) and the pain receptor neurons will have receptors for this Substance P

• Most of your internal organs DO NOT have these neurons

– You can’t really feel pain in many of your organs…but they can release Substance P

– “referred pain”…release of Substance P from a non-pain-innervated organ that triggers pain receptors in another region of the body

Pain receptors

Since many of your internal organs do not have free nerve endings that express or produce Substance P receptors, these organs do not have a direct “pain” receptor. These internal organs release Substance P and it characteristically triggers pain receptors in other areas of the body.

Special senses• Gustation: taste sensation

– “taste buds” = specialized clusters of chemoreceptors in your mouth

• NOT just on your tongue (you press candy or any other type of food onto the top or sides of your mouth…there are taste buds in those areas as well)

– Taste buds are not on the surface• Actually “folded” deep within the epidermal tissue that makes up

your tongue & oral cavity

– Only have 4 tastes: sweet, salt, bitter, sour• All your tastes are based from combinations of these 4 taste

sensations

– THERE IS NO MAP OF TASTES ON YOUR TONGUE!!!

• Your book is incorrect…this information has surfaced in the past 5 years…and it takes about 5 years to write a textbook

Gustation

3 things to remember:

1. You need saliva to carry the taste deep into these folds/papilla (ever try to taste something with a dry mouth?)

2. You have taste buds all over your oral cavity (not just on your tongue)

3. The 4 different tastes are distributed evenly throughout your tongue…you don’t have more sweet “buds” on the tip of your tongue

Special senses• Olfaction: smell

– Like taste, uses special chemoreceptors– In nasal cavity, retains these chemoreceptor

neurons within a specialized sensory structure• Nasal cavity is shaped to swirl incoming air up against

the top of your nasal sinus

• Chemoreceptor cells placed at the top of the nasal sinus will detect chemicals in the air

• Deliver neural impulses to brain

Olfaction

Olfaction

Special senses• Sight

– More specialized sensory structure• Eye, eylids, muscles to control the eye etc.

– Eye movement controlled by 3 separate nerves per eye

• Total of 6 muscles to control the movement of 1 eye• Very low ratio of nerve:muscle cells for very fine control

– Optic nerve carries nervous impulses from photoreceptors in the eye

– Total of 4 nerves devoted to each eye (3 for movement, 1 for the actual sight/optic signal)

• The bicep brachii muscle in your arm is innervated by 1 nerve that also controls a number of other muscles, as well as “senses” a large region around your shoulder

Special senses• Sight

– Eye itself is a very specialized sensory organ• 3 layers of special tissue

– Outer “sclera” that also contains the cornea

» Cornea is the primary “light refracting” or light “bending” part of your eye

» Largely specialized proteins, not many cells, and not many blood vessels

– Middle “choroid” layer (where many blood vessels are located)

– Inner “retina” layer

Special senses

Special senses• Sight

– Middle choroid layer has dark melanin pigment• To absorb light and keep it within the eye

– Inner retinal layer has rod and cone cells• Rods = light receptors• Cones = color receptors• Retinal layer is not physically attached to the choroid

layer– Actually pressed against the choroid layer by internal

pressure created by the volume of vitreous humor» Vitreous humor is one of the few stable parts in your

body (doesn’t change over your life…formed as you’re an fetus; what you have now will likely not change much in 10 years)

• Cornea/lens region filled with different fluid = aqueous humor (constantly being replaced)

Special senses• Sight

– Cones are clustered within a specific area of the eye• Fovea centralis (fovea)

• Color sensation is clustered in 1 area

– Rods are spread throughout the eye• Light sensation is dispersed, color sensation is “focused”

– Both rods & cones do not face the light…they face the pigmented choroid layer (away from the light)

• Light hits the choroid layer and “stays” there

• Rod & cone cells then sense this “retained” light and send impulses back through the optic nerve

– “blind spot” in the eye = where the optic nerve penetrates the eyeball/retinal layer

Special senses• Sight

– Rods & cones are not going to help if you cannot focus the image• Lens = focus• Remember that cornea = light bending• Light passes through the cornea, then through

lense– THEN, through vitreous humor and onto the

choroid layer

Eye

Special senses• Sight

– Once light/color is sensed by the rod & cone cells, the stimulate the visual neurons to fire action potentials• Only after enough color or light has stimulated

these cells will they release their neurotransmitter

– Similar to pain…the neurons themselves don’t directly sense light/color…the nearby cells tell them that there is light/color

– Signals are then relayed back to the brain via the optic nerve

The reason why light sensitivity requires so many more rod cells, rather than color sensitivity: for each optic nerve fiber/cell, there are many more rod cells that must influence the bipolar cells (signal concentration) to fire an action potential. The bipolar cells for light are less inclined to fire action potentials than those for color.

Optic Nerve & HemidecussationNote how about 40% of the light/color signals from the middle of your right eye will “hemidecussate” or transfer over to the left side of the brain. Also note how about 60% of the signals will remain on the right side of your brain

Special senses• Sight

– Whereas you can only taste 4 different types, you can normally see over 1000 different colors and shades

• 1000’s of different cone cell “subtypes” can sense different colors

• 10X as many rod cells as cone cells help you distinguish the shade or amount of light

– Remember that your brain puts all these signals together to tell you what you see

– Color blind = lacking cone cells of a particular color sensitivity

Special senses• Hearing

– Hearing & balance rely on the ear• Very complicated organ with multiple tasks

– Outer ear = pinna/auricle (cartilage)• Serves to “focus” the sound waves (air pressure) into

the auditory canal

– Middle ear = from the “ear drum” / tympanic membrane to the cochlea

• Acts to transfer sound in air into mechanical movement (solid)

– Inner ear = fluid filled organs• Cochlea = sound processing (sound into liquid

medium)• Semicircular canals & vestibule = balance/equilbrium

The middle ear contains the 3 smallest bones in your body: malleus (attached to the tympanic membrane/eardrum), incus & stapes. These 3 bones “articulate” with each other to amplify the sound pressure waves that reach the eardrum.

The middle ear also relies on equal air pressure between on both sides of the eardrum. This is why there is an auditory tube/estucian canal. This tube is connected to the back of your throat…ever get a sore throat that turned into an ear infection…ever have to “pop” your ears by opening your mouth?

Special senses• Hearing

– The cochlea is the region where sound pressure waves are converted to neural impulses• Pitch/tone = frequency of the wave• Intensity = volume (how high the waves are)

– Cochlea filled with fluid• 3 distinct “tubes” that wrap around forming a

coil (2.5 turns)

Remember that it is the movement of the basilar membrane in response to the frequency and amplitude of the pressure wave that defines the sound you hear.

Sound ProcessingOuter hair cells receive motor impulses from

brain. These cells can actually produce sound by themselves (when

stimulated by the CNS). This is in effort to “fine

tune” your sense of hearing.

Inner hair cells are loosely embedded in the

tectonic plate. It took over 50 years of controversy for

scientists to begrudgingly agree that the inner hair cells are

the true “hearing” cells.

Despite the differences in apparent function of these two “rows” of hair cells, they BOTH contribute to “hearing” or sound processing. One cannot function without

the other; outer hair cells can still transfer sound impulses as neural signals.

LOUD NOISES!!!!

The effect of loud noises and hearing loss = formation of scar tissue where hair cells used to be. You can “squish” the hair cells with enough sound pressure

(remember that the cells sense “sound” by getting squished by the pressure) that they can die! Interestingly/scary fact: despite the fact that loud noises can damage hearing, constant droning sounds (like the fan on your computer, an air conditioner

etc.) have a more long-term effect on hearing loss than a single rock concert!

Special senses• Equilibrium

– 2 regions of the inner ear involved in your sense of balance:

• Vestibule (contains utricle & saccule)• Semicircular canals

– All rely on hair cells similar the hair cells in the cochlea• Instead of pressure, these cells are embedded in special

“gels”– Vestibule gel has crystals of calcium carbonate

(otoliths)– Semicircular canals have “cupula” gel

• As your head moves, the fluid in these regions moves the respective gels, and this then moves the hair cells

The role of otoliths in “static equilibrium” = otoliths add mass that resists movement. If you tilt your head or jump up, these crystals will want to stay still/static. This drags the gel and pulls the hair cells.

Dynamic equilibrium = movement of fluid through the semicircular canals. As fluid moves one way through the respective canal, it will trigger the hair depolarize faster or slower. These hair cells always fire…moving the gel will trigger faster or slower stimulatory pulses to the nerve fibers.

Equilibrium• Placing the hair cells & different gels in strategic

locations in the vestibule allows complete equilibrium location– Utricle = tilting head anterior/posterior (nodding)

• Really more for horizontal movement; when you accelerate in a car

– Saccule = vertical movement (jumping up and down)

– The utricle & saccule are more responsible for STATIC equilibrium

• Because the gels have otoliths (crystals), they are pulled by gravity

– If you hold your head constantly sideways etc.

Equilibrium• Another set of receptor cells located in the

ampulla of each semicircular canal senses rotational movements– Hairs embedded into gel = cupula (no otolith crystals)

• As head spins, endolymph moves through canals and pushes/pulls on cupula

• Movement of cupula then flexes hair cells, triggering impulses

– Lateral semicicular canal = shaking head “NO”

– Anterior semicircular canal = nodding head “YES”

– Posterior semicircular canal = tilting head “Like…totally”

• Remember that the semicircular canals are responsible for dynamic equilibrium (telling you what’s going on when you MOVE).

Integration of the Senses• Recall that at least 80% of your sensory

information is taken in by your eyes (vision)– Despite your ear (semicircular canals &

vestibules) measuring equilibrium, your eyes also play an important role

• Watching a first-person movie like Star Wars, you sometimes feel like you’re moving

• Remember gustation involves your sense of olfaction as well– If you cannot smell well, you cannot taste as well

neither

Integration of the Senses• Where patients might lose a sense (hearing,

sight, smell)– They do not develop “super” senses with their

remaining senses• You only have what you’re born with (you can train

them to be more sensitive, but you cannot develop super hearing etc.)

– If you lose 1 sense, you have to place more emphasis and attention on your remaining senses

• They’re they same as before, you just focus on using them more effectively

Hearing defects• External otitis = outer ear infection• Middle ear infection = acute purulent otitis media

– Migration of infection due to common cold, flu, tonsillitus• In children, auditory canal is very short and straight (in adults,

auditory canal develops a curve)

– Infection and inflammation can close auditory canal• If pressure builds in middle ear, can rupture tympanic membrane

• Otosclerosis = ossification/fusion/degradation of ossicles

• Auditory deafness:– Conduction deafness = defect in outer or middle ear

(otosclerosis, infection etc.)• Sound is not conducted to the inner ear

– Perceptive/neural deafness = neural defect in inner ear

Cochlear implants• Surgical implant of a microphone, with

electrical leads that feed through the cochlea– If you have damaged hair cells, you can use the electrical

leads to trigger the neurons instead• Only used for patients with drastic hearing loss in BOTH ears

(monoaural patients not usually candidates)

– Where you can normally hear many different frequencies, a cochlear implant is limited to about 24 frequencies

• Voices, sounds tend to be very “robotic” and monotone

• Having more frequencies = more electrodes…run into trouble with electrode “bleeding” where 1 electrode triggers too many neurons

– Electrodes are placed in the cochlear duct (farther away from the actual neurons than the hair cells…at that distance, electrical impulses can “bleed” quite a distance)

Equilibrium defects• Meniere’s disease:

– Chemical change in the fluid of the inner ear• Vertigo, dizziness, hearing loss, tinnitus

• Often treated with surgery (remove a region of the inner ear…usually a region of the vestibule)

• Usually treated with diuretics (change blood volume = change in chemical composition of the fluid in the inner ear)